For example in an immunoassay, a sample in a container is tested for an immunoreaction first by placing a liquid such as physiological saline into the container and then placing a reagent thereinto. FIG. 24 shows a pipetting apparatus conventionally used for placing such a liquid and reagent (hereinafter referred to collectively as the "reagent") into a container.
The illustrated pipetting apparatus comprises a pipette table 90 for mounting a reaction container 7 thereon, a head drive device 9 mounted on the table 90 and a pipette head 91 attached to the output end of the drive device 9. The pipette head 91 is connected to a plunger mechanism (not shown), and has a pipette tip 92 fitted to its lower end and directed downward for drawing in and discharging a reagent.
The reagent is placed into each of cavities 71 of the reaction container 7 by controlling the drive device 9 by a controller 93 to bring the pipette head 91 close to the container cavity 71 and drawing off the reagent from the pipette tip 92 into the cavity 71.
When the reagent is to be diluted or mixed with another reagent 72 already placed in the cavity 71, the reagent to be added dropwise from the pipette tip 92 is brought into contact with the liquid surface of the reagent 72 within the cavity 71 to free the surface tension, whereby the reagent is drawn off.
If the pipette tip 92 itself comes into contact with the reagent 72 in the cavity 71 at this time, the reagent adhering to the tip 92 becomes mixed with other reagent in the subsequent pipetting step. Accordingly, the reagent within the pipette tip 92 needs to be drawn off with the extremity of the tip 92 positioned slightly above the liquid surface of the reagent 72.
Pipette tips of different capacities are used for applying the reagent in different quantities of 0.5 .mu.l to 200 .mu.l by the pipetting apparatus. Prepared for the apparatus are a pipette head of small inside diameter for mounting pipette tips having a small capacity of 0.5 .mu.l to 10 .mu.l, and a pipette head of large inside diameter for use with pipette tips having a large capacity of 5 .mu.l to 200 .mu.l, and these pipette heads are alternatively used by replacement.
With the conventional pipetting apparatus, however, the pipette head is fixed to the apparatus main body by fastening means as by screwing. The apparatus therefore has the problem that the procedure for replacing the pipette head is not only cumbersome but also difficult because the head is as small as about 20 mm in outside diameter.
When the reagent is to be diluted or mixed with the reagent 72 in each cavity 71 of the reaction container 7 as stated above, the pipette head 91 is lowered to position the pipette tip 92 as close to the liquid surface of the reagent 72 as possible, whereas since the liquid level of the reagent 72 differs from cavity to cavity, the liquid level in each cavity must be measured to adjust the level of the pipette tip 92. At this time, an accuracy of about 0.1 mm is required of the measurement of the reagent liquid level.
Although it is advantageous to use a laser measuring instrument from the viewpoint of accuracy of distance measurements, the instrument is not usable for transparent reagents or reagents which are likely to undergo a photochemical reaction since the reagent is irradiated with a laser beam in this case.
Accordingly, it is practice to measure the distance to the liquid surface of the reagent 72 by an ultrasonic sensor attached to one side of the pipette head 91 and to feedback the measurement to the control of the head drive device 9. The sensor emits ultrasonic waves toward the object of measurement, receives the ultrasonic waves returning upon reflection at the object and measures the time taken for the emitted waves to return to the sensor to determine the distance to the object based on the time measurement.
While the level of the pipette head is controlled according to the distance thus measured by the ultrasonic sensor, the pipette tip 92 is likely to become damaged when an erroneous movement is involved in measuring the distance for one cause or another and if the pipetting operation is continued based on the result of measurement then obtained. For example, in the case where the measurement obtained is greater than the distance to the bottom face of the cavity 71 of the reaction container 7, the controller for the head drive device 9 sets as a target position for lowering the pipette head 91 a position where the extremity of the pipette tip 92 will be located further below the bottom face of the container cavity 71, with the result that when the head is lowered, the extremity of the tip 92 comes into striking contact with the bottom face of the cavity 71, bending the end portion and rendering the tip unable to draw off the reagent. The impact resulting from the striking contact of the pipette tip 92 with the reaction container 7 is also likely to cause damage to the pipette head 91.